Nearly all of L. Sprague de Camp's sf work tends toward humor, whether it be irony, satire, or outright slapstick. And, of course, there's Eric Frank Russell, who often specialized in humorous sf; most of his best is in short story form, and his stories have been collected together in large omnibus volumes now that should be available through libraries or by order. As with Pratchett and some of the others, there's an underlying serious point to much of Russell's work, but the stories can be enjoyed just for the humor itself.
